The present invention relates to the formation of contacts through insulating layers in integrated circuits, and more particularly, the invention relates to eliminating the development of voids in the contacts when openings in the insulating layer are filled with metal to form the contacts.
In a typical integrated circuit fabrication technology, after transistors are formed on a semi-conductor substrate, a layer of dielectric material is used to coat the surface of the transistors to physically and electrically insulate them. Once this dielectric material is deposited, openings are etched through the dielectric material to the underlying semiconductor substrate. Conductive material is deposited into these openings in order to make electrical contact to the substrate surface. These openings filled with conductive material are called contacts.
As the density of the active devices formed on the substrate increases, the width of the contacts decreases. However, the thickness of the insulating layer through which the contacts extend can only be decreased to a certain minimum thickness. An insulating layer that is too thin results in an intolerably high interlevel capacitance, which ties up otherwise available conduction carriers. As dimensions continue to be scaled down to the submicron and nanometer level, the width of the contacts decreases in size but the depth of the contacts (through an insulating layer) remains the same. As a result, the aspect ratio of the contacts, and of the openings in which the contacts are formed, increases as circuitry becomes more densely packed.
The contact fill, which commonly uses tungsten, gets more challenging together with contact dimensions scaling down, and the need for liner silicide to achieve low Rcontact. Mostly due to an overhang of metal at the top of the opening in which the contact is made, there is a formation of a large void, referred to as a key hole, in the contact, later causing a tungsten contact-to-copper contact reliability and/or open issue.